Hydrogen plays an important role in dramatically changing materials properties. Hydrogen embrittlement of steel is a well-known example of the detrimental nature of hydrogen on materials properties. In other cases, hydrogen is introduced intentionally into materials for hydrogen storage and also in passivating bulk defects in semiconductors. The ability to directly image the 3D distribution of hydrogen in solids at nanoscale is highly valuable to reveal the mechanisms by which hydrogen influences the physical and chemical properties of materials. The difficulty in the characterization of hydrogen stems from the fact that it is a light element and hence the typical methods of chemical mapping such as Energy-Dispersive X-ray Spectroscopy (EDX) are not suitable. Furthermore, hydrogen in certain materials (e.g. metallic alloys) is highly diffusible at ambient temperature and hence poses another analytical challenge. For these reasons, nanoscale time-resolved 3D mapping of hydrogen in relatively large volumes (> 1000 um3) is still lacking. The techniques currently available to image hydrogen distribution are mainly autoradiography, silver decoration, Scanning Kelvin Probe Force Microscopy, Secondary Ion Mass Spectrometry (SIMS) and Atom Probe Tomography (APT). The individual techniques have their strengths and limitations in terms of the spatial and time resolutions and fields of view1. Among these techniques, SIMS and APT are the most promising methods for high-resolution 3D imaging of hydrogen. SIMS is well-known for high-sensitivity analysis of relatively large areas (tens of um) but the spatial resolution is limited to ~50 nm in commercial SIMS instruments (Cameca NanoSIMS). To correlatively overcome this limitation, we have recently demonstrated in-situ SIMS in a Transmission Electron Microscope (TEM) in which the SIMS images are correlated to high-resolution structural images obtained from TEM7. In parallel, our group has also demonstrated a much improved SIMS image resolution of ~15 nm using the high-brightness Gas Field Ion Source (GFIS) used in Helium Ion Microscopy (HIM). Nevertheless, the SIMS image resolution is fundamentally limited to ~10 nm set by the ion-solid interaction volume. Another limitation is that the quantification of SIMS data is difficult because of the matrix effect. In comparison, APT offers excellent spatial resolution (sub-nm to few nm) and the chemical composition can be quantified. However, the main limitations for APT are that the sample size is very small (needle length of ~ 100 nm and tip radius < 10 nm) and sample preparation is laborious. By combining SIMS-based correlative microscopy and APT imaging techniques, the limitations of the individual techniques can be overcome and length scales ranging from tens of micrometre to nanometre can be analysed, covering over 4 orders-of-magnitude. In this project, we plan to address the analytical challenges associated with the high-resolution imaging of the 3D distribution of hydrogen in mesoscale and nanoscale volumes mainly by SIMS-based correlative microscopy alongside complementary APT for a few selected cases. The project will commence with samples in which hydrogen is immobilized and then progressively shift towards more complex samples containing highly mobile hydrogen which will require analysis under cryogenic conditions. Furthermore, the datasets from the multiscale multimodal analysis will be integrated to correlate chemical and structural characteristics (e.g. dislocations, grain boundaries, nano-precipitates, etc) and to identify and corroborate underlying materials phenomena. We have selected case studies comprising of varying degrees of difficulty from three most prominent domains of materials science involving hydrogen-related research: (i) hydrogen in structural alloys (e.g. steels), (ii) hydrogen storage materials and (iii) hydrogen in semiconductors.